1. Field of the Invention
This invention relates to the manufacture of semiconductor devices by employment of thermal gradient zone melting and, more specifically, to a method of enhancing the magnitude of the thermal gradient established in the semiconductor material by the temperature gradient zone melting process.
2. Description of the Prior Art
In the manufacture of semiconductor devices, it is normally necessary to alter the conductivity type of a selected region or regions of a semiconductor body by doping these regions with conductivity-modifying impurity atoms. Today, such doping is usually accomplished commercially by solid state diffusion, ion implantation, liquid epitaxial growth, or vapor epitaxial growth. Such factors as cost, speed, junction characteristics and the particular type of semiconductor material being used determine which method is most practical.
A little-used and less widely known technique for doping semiconductor material is temperature gradient zone melting. This technique can produce very abrupt junctions with unusual configurations and high dopant concentrations is a body of semiconductor body in a relatively short period of time. Early descriptions of temperature gradient zone melting and some of its applications are found in U.S. Pat. No. 2,813,048 issued to W. G. Pfann and in his book Zone Melting copyright by John Wiley and Sons, Inc. While the basic principle of temperature gradient zone melting was known very early in the life of the semiconductor industry, number of unsolved problems have prevented its use as a standard processing technique by the semiconductor industry.
Temperature gradient zone melting is a process in which a small amount of a dopant metal and/or an intrinsic metal is placed on a selected surface area of a body of semiconductor gradient at an elevated temperature. The overall temperature at which the process is carried out must be sufficiently high that a zone or droplet of alloy melt containing both the semiconductor material and metals will form. Under these conditions, the melt zone will migrate along and up the lines of heat flow from low temperature to high temperature leaving in its path a recrystallized region of semiconductor material containing therein the solid solubility limit of the dopant. The migration speed of the molten zone is directly proportional to the magnitude of the thermal gradient. Thus it is advantageous to have as high a temperature gradient as possible to reduce processing times. The temperature gradient must also be uniform and unidirectional if the pattern of dopant metal imposed on the entrance face of the wafer is to be reproduced as a recrystallized dopant zone faithfully in the semiconductor body. The distorting effects of stray thermal gradients perpendicular to the desired migration direction are minimized by a relatively large thermal gradient along the migration direction since the resulting faster processing speed implies that there is less time for the stray perpendicular thermal gradients to cause irregularities to develop.
One of the most difficult problems blocking the widespread use of temperature gradient zone melting has been how to generate a large uniform thermal gradient in a thin fragile semiconductor wafer without mechanically stressing the wafer or contaminating the wafer with undesirable impurities.
A number of means of applying a large uniform thermal gradient have been tried including a plasma torch, a gas flame, a scanning electron beam, a heated anvil and infrared radiation. Each of these methods has its own problems and none of these methods produces both a uniform and a large thermal gradient. For example, while the plasma torch, gas flame, scanning electron beam and heated anvil can produce large thermal gradients, these same thermal gradients have not been uniform and have caused irregularities in the doped zones produced by temperature gradient zone melting utilizing these techniques.
The most practicable means of imposing a uniform thermal gradient on a semiconductor wafer is by exposing one of its major surfaces to a widely dispersed, uniform, intense source of radiation such as infrared or optical radiation that is absorbed at the exposed surface of the wafer. The absorbed heat then flows uniformly through the wafer where it is reradiated to a cold black heat sink facing the other major surface of the wafer.
The most satisfactory method of applying a uniform temperature gradient to a thin semiconductor wafer is by exposing one side of a semiconductor wafer to an intense uniform source of infrared radiation and the opposing side of the wafer to a cold black body. For a complete description of the infrared radiation method, attention is directed to the copending application of John K. Boah entitled "Temperature Gradient Zone Melting Utilizing Infrared Radiation", application Ser. No. 578,807, filed May 19, 1975, assigned to the same assignee as this application.
Although the infrared radiation method has produced the most uniform thermal gradients of all the above listed methods, it also produces the smallest thermal gradient of the above methods. This small thermal gradient is the result of several factors. First, the intensity of available infrared radiation sources is limited. Secondly, the large reflectivity of semiconductor wafers caused by the unusually high refractive index of semiconductor materials limits the amount of radiation the semiconductor wafer can either absorb or re-radiate. Since the magnitude of the thermal gradient in the wafer is directly proportional to the amount of heat absorbed by the wafer, the limited absorbtivity of a semiconductor wafer as well as the limited radiation sources available imply in turn a relatively small thermal gradient in the semiconductor wafer. This small thermal gradient, in turn, causes temperature gradient zone melting speeds to be low, increases the chance of doped zone irregularities produced by competing lateral thermal gradients and makes impossible the dopant of zones by temperature gradient zone melting below a critical dimension that is inversely proportional to this applied thermal gradient.
In summary, although the infrared radiation method of temperature gradient zone melting makes temperature gradient zone melting practicable on the commercial scale by providing a means of producing a uniform thermal gradient, it has several drawbacks since it produces a relatively small thermal gradient in a semiconductor body.